To install a new window, remove the old window, prepare the opening for the new window, inspect the window stop, dry fit the new window and then install it. Add installation, jambs and molding to finish the job.
To remove the old window, start by removing the trim and jamb extension. Unscrew the old window from the jamb and remove it with care.Once the window is out of the wall, inspect the jamb for damage. If there is damage, repair or replace the jamb before putting the new window into to wall. It is also important to inspect the window stop, which is the wood on the exterior side of the jamb that stops the window from moving out of place. The stop may be fine to reuse, but if it is not in usable condition, replace it.
Before using silicone to secure the window, dry fit the window to ensure that it fits into the space. Adjust the space until the window fits into the opening. When ready, run the silicone along the stop, and slide the window into place. If the window came with hardware, insert the screws at this point. Add insulation, if necessary, before installing the extension jambs and trim molding. To finish, add the final touches, such as paint or stain.